Home wireless networks or PANs (Personal Area Networks) are traditionally designed to interconnect communications devices, for example digital instruments, telephones, personal digital assistants, speakers, television units, multimedia players situated in proximity to the user. The range of a communications network of this kind is of the order of a few meters. Such a network can also be used to make the different personal devices communicate with one another (this is commonly known as intra-personal communications) or to connect them to applications supported by the Internet for example.
Home networks may be wired (as is the case for USB type and Ethernet type networks or again according to the IEEE 1394 standards) but may also rely on the use of a wireless medium. The term used then is wireless home network or wireless personal area network (WPAN). The Bluetooth (IEEE 802.15.1), UWB, ZigBee (IEEE 802.15.4), IEEE 802.11e or IEEE 802.15.3 standards are to date among the most widely used protocols for networks of this type.
Such protocols generally provide for two types of access to the shared wireless medium.
A first type of access is that of the collision detection mode (also called CSMA/CD or Carrier Sense Multiple Access/Collision Detection). This type of access allows each of the devices of the communications network to manage its sending operations as a function of its needs and the availability of the medium. When there is no information to be transmitted, the device receives data packets that travel on the medium. When this device needs to send one or more data packets, it ascertains that no frame has been sent on the medium. If this is the case, it can send its data packet. If this is not the case, it awaits the end of the transmission in progress. Since the method of access is that of collision detection, when sending, a device may detect a problem of contention and stop in order to re-send its data packet subsequently, i.e. when it has the floor again. In order to minimize the risk of undergoing a second collision with a same device, each device of the network waits for a certain period (which may be a random period) before attempting a new operation of transmission. However, so as not to saturate a network which might be already highly loaded in terms of communications, a device of the network does not try indefinitely to retransmit a packet of data if, at each attempt, it is in a state of conflict with another device of the network. Thus, after a certain number of unsuccessful tries, the data packet is eliminated, thus preventing the collapse of the network. The higher layers are then alerted to the failure of transmission of the message.
A second type of access is a time division multiple access (TDMA) mode. This second mode of access is a multiplexing mode used for the transmission of several signals on one and the same communications channel. This is time multiplexing, the principle of which is that of subdividing the available time into several time slots or speech times which are successively allotted to the different devices of the network.
Networks of this kind rely classically on the presence of a master device responsible for setting up connections of the network, synchronizing the speech times or time slots of each of the devices of the network and arbitrating access to the shared wireless medium.
Radio transmission systems currently use a wide range of transmission frequencies, generally from 2.5 GHz and 60 GHz. These frequencies are particularly well suited to data transmission at very high bit-rates in a limited range, for example as means of connectivity between the different elements of a “home cinema” type communications network. Indeed, in this case of use, the range is limited to about ten meters. However, the bit rates brought into play are very high (over one gigabit per second or Gbps or Gbit/s) owing to the nature (audio, video) and very high resolution of the information transmitted.
The use of transmission and reception antennas may furthermore play a crucial role in the quality of communications for wireless home networks of this kind.
An isotropic antenna, i.e. an antenna radiating with the same physical characteristics in every direction of space, is a theoretical model that cannot be made in practice. Indeed, the energy radiated by an antenna is in reality unequally distributed in space, certain directions being more favored. The term used then is “radiation lobes”. A radiation pattern of an antenna can be used to view these radiation lobes in all three directions of space, i.e. in the horizontal plane or in the vertical plane including the most important lobe. The proximity and conductivity of the ground or of the conductive masses surrounding the antenna may furthermore have a major influence on the radiation pattern.
The directivity of an antenna in the horizontal plane is a major characteristic in the choice of an antenna. Indeed, an omnidirectional antenna radiates in the same way in every direction of the horizontal plane while a directional antenna for its part has one or two lobes that are appreciably more important than the others. The term used then is “major lobes”. It must be noted that an antenna is all the more directional as the most important lobe is narrow. The directivity corresponds to the width of the major lobe, between the attenuation angles at 3 dB. The gain of an antenna is then defined as the increase in the power sent or received in the major lobe relative to an omnidirectional antenna.
A signal processing based on the directivity of the antennas (known as beam-forming) is a technique that relies on the use of transmission tables or reception centers which control the orientation and/or the sensitivity of an antenna as a function of a radiation pattern. At the reception of a radio signal, this technique is used to increase the sensitivity of the receiver device in a desired direction and thus reduce the sensitivity of the antenna for areas of interference or highly noisy areas. During the transmission of a radio signal, the technique of antenna directivity increases the power of the radio signal in a desired direction.
Although such communications systems are advantageous from the viewpoint of their installation and their particularly high applications bit-rates (allowed for high frequencies such as in the 60 GHz frequency band for example), they are highly sensitive to interference and shadowing of radio links communications caused for example by obstacles. Thus, the relevance of the use of communications links between the different devices of the network is highly correlated with the positioning and shifting of obstacles in the network, which are all so many sources of dynamic masking for the network.
A prior-art technique, described in the European patent application EP1406466 presents a technique for tracking a mobile device in a communications network. It consists more particular in tracking the mobile device by detecting successive positions in different sub-areas of the network.
However, such a method assumes the sending by the mobile device (whose position has to be determined) of a piece of information to the detection devices of the network so that they determine its position in the network. Such a technique furthermore does not meet the problem of locating a disturbing obstacle, which is distinct from the devices of the network.